Problem: The arithmetic sequence $(a_i)$ is defined by the formula: $a_1 = 9$ $a_i = a_{i-1} - 2$ What is $a_{10}$, the tenth term in the sequence?
Explanation: From the given formula, we can see that the first term of the sequence is $9$ and the common difference is $-2$ To find the tenth term, we can rewrite the given recurrence as an explicit formula. The general form for an arithmetic sequence is $a_i = a_1 + d(i - 1)$ . In this case, we have $a_i = 9 - 2(i - 1)$ To find $a_{10}$ , we can simply substitute $i = 10$ into the our formula. Therefore, the tenth term is equal to $a_{10} = 9 - 2 (10 - 1) = -9$.